Solvent flood with expanding oil phase



r 3,267,999 Ice Patented August 23, 1966 3,267,999 SOLI/ENT FLGE WITHEXPANDNG @lL PHASE Ronald L. Reed, Austin, Tex., and .Ioseph 5. Taber,Cheswick, Pa., assignors to Golf Research & Deveiopmcnt Company,Pittsburgh, Pa., a corporation of Delaware Filed Mar. 18, 1963, Ser. No.265,931 Claims. (Cl. 1613-9) rIhis invention relates to the recovery ofpetroleum by an improved miscible displacemen-t process.

`In the production of petroleum by primary recovery methods involvingthe displacement of oil by natural reservoir energy, total recovery isgenerally significantly less than percent of the oil initially in place.The great volume of presently unrecovered petroleum and the increasingcost of petroleum exploration have motivated a Search for recoverymethods characterized by high displacement efliciencies and economicpracticability. Of the many recovery schemes proposed, the conventionalwater hood is often economically attractive. Unfortunately, Water ilooddisplacement eiciencies are among the lowest for the lavailablesecondary recovery methods and result in the bypassing of an appreciablevolume of oil, which remains in the :flooded reservoir. Recovery methodsinvolving continuous injection of a solvent would displace the entireVolume of reservoir oil, if injection of the solvent were continuedindeiinitely but the cost of such a process is prohibitive because ofthe large amoun-t of solvent used and left in the reservoir at the endof the ilood.

The miscible slug process, which combines the advantages of Waterilooding 4and solvent ooding, consist of displacing the oil with apredetermined volume of solvent which is driven toward .the producingwells by a Water ilood. yThe recommended solvents, herein referred to asamphipathic solvents, are liquids which are miscible with 'water insubstantially .all proportions and are also miscible -with oil insubstantially all proportions. Amphipathic solvents which have beensuggested and are available at prices allowing their use in a miscibleslug process are, however, miscible with mixtures of oil and water onlyin a narrow range of compositions. Hence, even so-called miscible slugprocesses involve a two-phase ow of oil and an aqueous phase fromreservoirs initially containing an aqueous phase.

The requisite size of the solvent slug is a major economic factor anddepends upon the iluid dynamics and phase behavior of the system. Fluidviscosity differences and reservoir heterogeneities cause mixing at theliquid inter- 'faces and the formation of an oil-brine-solventtransition zone `and a solvent-water transition zone at the leading .andtrailing edges, respectively, of the solvent slug.

'Man-y methods have been suggested for adjusting the viscosities at theleading edge of the solvent slug to assure a favorable viscosityrelationship. One such method proposes injecting a slug of lighthydrocarbon, such as LPG, kerosene, or naphtha, ahead of the -solvent toreduce the viscosity of the oil that is contacted by the solvent slug.Although `this device provides a favorable viscosity ratio and in areservoir devoid oi water would result in a miscible displacement of theoil, it does not cause m-iscible displacement :by the solvent of theiluids from reservoirs containing Water because lthe three componentswater, oil, and solvent are miscible in only a narrow range ofconcentrations and the presence of reservoir 'water causes the liquidsystem to form two phases. Furthermore, the injected hydrocarbon, beingsubstantially completely immiscible with water present `in thereservoir, twill have no effect of displacing connate Water from thereservoir and the net effect of the injection ot the hydrocarbon will besubstantially to substitute injected hydrocarbon tor the oil originallyin the reservoir at the trailing edge of the stabilized ban-k. Hence,even rwith a favorable viscosity ratio at the dood front, displacementeiiiciency is determined by .the phase behavior of the reservoir liquidswith the amphipathic solvent in the transition zone at the leading edgeof the solvent slug.

This invention resides in a process yfor the recovery of oil lfrom areservoir containing an aqueous phase, which comprises injecting amixture of a solvent, which is miscible with the oil and with water, anda hydrocarbon liquid into a reservoir ahead of a pure solvent slug andfollowing the solvent slug with Water to displace oil and the injectedliquids through the reservoir to a production wel-1. `In one embodimentof this invent-ion a slug of a low molecular weight oil such as LPG,gasoline or naphtha is displaced through the reservoir ahead of themixture of :solvent and hydrocarbon liquid.

IFIGURE 1 is a ternary phase diagram yfor a system of liquids comprisingisopropyl alcohol, isooctane, and two percen-t calcium chloride brine.

`FIGURE 2 is a ternary phase diagram for a system of liquids comprisingisopropyl alcohol, isooctane, and two percent calcium chloride brineshowing the path followed by the composition of the uids in thereservoir during the process of this invention.

FIGURE 3 shows a comparison of typical brine production histories lforexpanding aqueous phase .and expanding oil phase miscible floods.

It is characteristic of a miscible slug process in reservoirs containingoil and brine that, as the injected liquids advance through thereservoir, `four distinct zones are lformed and move toward theproducing wells. The initial ilowing zone consists of pure oil becausethe connate brine in this region is discontinuous and hence has nomobility. In the event the reservoir has been water flooded to theresidual oil saturation, the initial zone is the reservo-ir Water. -Ineither event, the initial zone is )followed by a stabilized zone of twophase flow in which oil .and brine ow in almost constant proportion.This is followed by the transition zone in which the solvent dissolvesin the oil and in the brine, and the proportions of brine phase and oilphase iloyving vary as the solvent concentration increases. Eventually,the three liquids achieve miscibility with the formation of a singleflowing phase which constitutes the fourth zone. `With respect to thisinvention, a description of succeeding fluids in the dood is ntpertinent, and the fourth zone may be treated as one in which `thesingle phase of miscible liquids behind the transition zone grades intop-ure solvent and subsequently into Water.

The ternary diagrams of FIGURES ll and 2 illustrate the phase behaviorof the liquids in the transition zone and in the miscible zone ahead ofthe pure solvent. Throughout this application, all references to ternaryd-iagrams shall be to diagrams showing percent 'water at the lower leftangle, 100 percent solvent at the upper angle, an-d 1100 percent oil atthe lower right angle of the ternary diagram. rFIGURE 1 represents aconventional miscible slug process in which va solvent slug of isopropylalcohol is displaced by a water drive through a reservoir containingisooctane and calcium chloride brine. Mixing at the leading edge of thesolvent slug will form an oil-brine-solvent transition zone precedingthe pure solvent. At a point in the reservoir just ahead of thetransition zone, i.e., in the stabilized zone, oil and brine are atconcentrations indicated by point A on the oil- 'brine side ofthediagram. As the transition zone passes this point in the reservoir, themixture of solvent, oil, and brine follows the composition path of theline AC, and phase relations in the transition zone are indicated by therelation between the composition path line segment AB and the tie-linessuch as EF. For example, when liquid concentrations in the transitionzone are indicated by point D, the ratio of the volumes of oil phase tobrine o phase equals the ratio of the tie-line segments ED to DF. As theliquids approach the miscible region, the tie-line segments representingthe proportion of oil phase decrease in length .and vani-sh asmiscibility is achieve-d. This occurs because the oil dissolves thebrine phase as solvent concentration increases, and in an equilibriumsystem, as is represented by the ternary diagram, the brine phasecontinues to grow until the oil phase disappears when the system has thecomposition B at which line AC crosses the binodal curve. Because of theeventual dominance of the brine phase, such processes are referred to asexpanding aqueous phase miscible slug processes.

A significant feature of the expanding aqueous phase process is that, asoil dissolves in the brine phase, the oil volume in the reservoirdecreases until the oil phase becomes discontinuous. The oil phaseremains discontinuous during the rest of the flooding. When thiscondition exists, the reservoir permeability to oil vanishes, and theoil phase is present behind the transition zone in isolated gangliawhich remain immobile in the reservoir while the continuous brine phaseis displaced. Thus the expanding aqueous phase reduces oil recoverybecause of the volume of oil that is left in the reservoir.

Misc-ible flooding processes exhibit an expanding aqueous phase when`the liquid mixture in the transition zone follows a composition pathline that intersects the binodal curve at a point to the left of theplait point as shown in FIGURE l. The compositi-on path line, AB, inFIGURE 2 indicates that, if the liquids lin the transition zone achievemiscibility through a region lying to t-he right of the plait point, thetie-line segments such as DF, representing the proportion of brinephase, eventually vanish. In such a system the brine phase becomesdiscontinuous, and the oil phase remains continuous and mobile as theliquids achieve m-iscibility. A process with these characteristics isreferred to as an expanding oil phase misci-ble slug process.

Sol-vents having the desired miscibility, with oil and als-o with waterare in most instances low molecular weight, oxygenated organic liquids.Of these compounds, the monohydroxy alcohols having from 1 to 3 carbonatoms per molecule are preferred amphipathic solvents because of theiravailability and relatively low cost. Methyl alcohol, and to a lesserextent ethyl alcohol, are miscible only with hydrocarbons of lowmolecular weight. Their usefulness in the process of this invention is,therelfore, limited to the embodiment of this invention in which themixture of solvent and hydrocarbon liquid is preceded by a slug of a lowmolecular weight hydrocarbon which is miscible with those alcohols.Isopropyl alcohol is the preferred alcohol. Because the alcohols listedabove -form a system with brine and crude oil in which the plait pointis to the right of the binodal peak, the path of the composition of thesystem as the amphipathic solvent is mixed with the water-oil mixture ofthe stabilized zone passes to the left of the plait point and, hence,results in an expanding aqueous phase, as shown in FIGURE l. Thecharacteristics of other amphipathic solvents which are miscible witheither water or oil such as acetone, dioxane, acetaldehyde, and ethyleneoxide .are similar in resulting in an expanding aque-ous phase. Tertiarybutyl alcohol also can be used and will result in a system having aplait point to the left of the peak of the binodal curve in systems witha `few reservoir oils and water. However, the high cost of tertiarybutyl alcohol will almost always preclude its use in a miscible slugprocess even in those systems which result in a plait point -to the leftof the binodal peak. This invention, in which a liquid hydrocarbon isadded to the amphipathic solvent, is useful in miscible slug processesusing any yof the solvents mentioned above, which result in the plaitpoint of the system being to the right of the binodal peak.

In this invention, an expanding aqueous phase process is converted intoan expanding oil phase process by injecting a slug of solvent mixed witha hydrocarbon ahead of the pure .amphipathic solvent. Reference to puresolvents is to distinguish lthe solvent slug from the solvent to whichhydrocarbons are deliberately added according to the method of thisinvention. Solvents, or mixtures of solvents containing impurities oreven incidental amounts of hydrocarbons are referred to as pure solventsin this specication. FIGURE 2 is a ternary diagram for the iso- -octane,`calcium chloride brine, 4and isopropyl alcohol system showing the pathof reservoir huid composition through the transition zone when displadby a slug of isopropyl alcohol mixed with isooctane followed by a slugof pure isopropyl alcohol. This diagram indicates that when thetransition zone contacts the reservoir liquids at concentrationsindicated by point A', the added hydrocarbon gradually increases the oilconcentration, and the liquids are forced `to approach miscibility alonga composition path that intersects the binodal curve at B' to the rightof the plait point. As miscibility is achieved in this process, thetie-line segments such as DF representing the proportion of brine phasein the transition zone, decrease in length and ultimately vanish. Thisindicates that the brine dissolves in the oil phase as solventconcentration increases. Ultimately, the brine phase volume decreases tothe point that the brine phase becomes discontinuous and reservoirpermeability to brine does not exist. The brine phase remains behind theflood front in isolated ganglia, and the oil phase expands in volume andconstitutes the sole, continuous ilowing phase as miscible displacementis achieved. This results in increased recovery of oil from thereservoir. Furthermore, the higher concentration of brine behind theflood front reduces the reservoir resistance to ow of water, and thescavenging water flood can be conduc-ted at injection pressures lowerthan those for an expanding aqueous phase process.

A wide variety of hydrocarbon liquids can be mixed with the amphipathicsolvent for injection into the reservoir ahead of the solvent slug toachieve the desired expanding oil phase. One suitable liquid hydrocarbonthat can be mixed with the solvent is Ithe reservoir oil. In mostinstances, however, it is preferred to mix hydrocarbon oil of lowermolecular weight that the reservoir oil wit-h the solvent. Thehydrocarbons of lower molecular weight tend to lower the binodal curvefor the resultant 4-cornponent system `and thereby reduce the amount ofsolvent needed to retain miscibility during the subsequent injection ofthe pure solvent and lthe following water drive. Suitable hydrocarbonsof lower molecular weight than the reservoir oil are kerosene, naturalgasoline, and LPG. Any broad or narrow cut of these hydrocarbons issuitable. Highly preferred liquid hydrocarbons for this process arehydrocarbons containing high concentrations of aromatic hydrocarbons.Suitable highly aromatic hydrocarbons are benzene and platformate, areformed gasoline fraction. The highly aromatic hydrocarbons haveadvantage over other hydrocarbons of tending to shift the plait point ofthe resultant 4component system to the left thereby reducing the amountof hydrocarbons required.

The liquid hydrocarbon is admixed with the solvent in a predeterminedamount designed to induce a path of the reservoir fluids compositionwhich passes to the right of the plait point. Referring to FIGURE 2 ofthe drawings, the injected mixture should have a concentration of oilslightly in excess of that indicated by the intersection of a line fromA through the plait point with the side of the ternary diagramconnecting the percent solvent and 100 percent oil vertices. A lineconstructed to indicate the proper oil concentration would be one suchas line A'G'. The concentration of the hydrocarbon required to theinjcated-solvent-hydrocarbon mixture can be determined by a series ofbottle tests in which samples of reservoir oil and water typical ofstabilized zone composition are mixed with hydrocarbon-solvent solutionsof increasing hydrocarbon concentration until the concentration ofhydrocarbon required to cause an expanded oil phase is determined. 'Ihesizes of the slug of the mix- .ture of hydrocarbon oil and solvent andthe following slug of pure solvent are designed to avoid completebreakdown o-f the pure solvent slug until it traverses a surfiicientdistance in the reservoir to obtain the benefits of this inventionthrough the entire reservoir volume swept by the flooding process. Aslu-g having -a volume of 2 to 20 percent of the reservoir pore volumeordinarily should lbe used. The size of the slug of mixed solvent andhydrocarbon liquid, and of solvent, is determined for each reservoir oilby preliminary core tests.

The improvement in displacement efficiency resulting from an expandingoil phase process is illustrated in FIG- URE 3, which presents acomparison of a brine production histories as a function of cumulativeliquid production from the reservoir for the expanding aqueous phase andthe expanding oil phase processes. The upper curve, shown as a solidline, applies to the expanding equeous phase process and shows thefraction of brine, fw, in the flowing liquids -for each stage of therecovery process. In the zone of pure oil flow, fw is zero, but in thestabilized zone the fractional ow of brine increases rapidly toapproximately 70 percent, with oil comprising the remaining 30 percentof the owing liquids. The transition zone is characterized by Itheappearance of solvent dissolved in the ilowing oil and bine phases andby marked changes in the conditions of flow. From this point onward thefw curve represents not the fractional flow of :brine in the volume ofoil and brine flowing but rather the -fractional ilow of the brinesolution in the total flowing volume. For this expanding aqueous phaseproce-ss, the increase of the value of fw to 100 percent when only thebrine phase is iiowing in the miscible region reects the shrinkage ofthe oil phase with the consequent loss of continuity and oil mobility.

The lower curve in FIGURE 3, shown as a broken line, illust-rates thebrine production history of an expanding oil phase process and theimprovement in displacement of efficiency resulting from this process.The fw curve did not become `greater than zero until a greatercumulative production of oil was attained, indicating Va larger volumeof the pure oil zone. In addition, fractional ow of brine in thestabilized zone was `only approximately 58 percent and the flow of oil,approximately 42 percent. In the transition zone, fractional flow of thebrine phase decreased to zero, indicating Ithat the oil phase hasexpanded as the continuous, mobile phase while the brine phase remainsimmobile behind the flood front. Sustaining a mobile oil phase causesincreased flow of oil in the transition zone with a high oilconcentration at the leading edge, and part .of Ithe reservoir o-il`ahead of that zone forms a bank that increases in length and results ina larger volume of oil ilowing in the stabilized and pure oil zones. Inthis manne-r a larger Volume of oil is displaced from the reservoirvolume contacted by the solvent slug.

The improvement in oil recovery resulting from this invention isindicated in Table I. The two runs reported in Table I where made in aBerea sandstone core that was 35 feet long and satura-ted with isooctaneand two percent calcium chloride brine. Initial saturation of the coreswas performed by evacuating land `admitting deaerated water into thecore until full. High oil saturations (.i.e., irreducible Water to anoil flood) were achieved by flowing three to ve pore Volumes of oilthrough the Water-saturated core until water product-ion ceased.Residual oil saturations were obtained by Water ooding the core at thisstage. Run 1 represents a conventional miscible slug process in w'hich aslug of pure isopropyl alcohol is followed by a Water drive. It iscustomary to measure slug size -by the length of the slug in the core.In Run 1, the slug of solvent was 5.2 feet long, corresponding toapproximately percent of the pore volume. This slug traveled a distanceof 23 feet, or lapproximately 66 percent of the total length of the corebefore mixing caused the entire solvent slug to be consumed in theformation of two phases. Oil recovery by this process was 79 percent ofthe original oil volume.

In the expanding oil phase process Iof Run 2, a 1.3 foot slug of p-ureisopropyl alcohol was preceded by a mixed slug of isooctane (thereservo-ir oil) and isopropyl alcohol, in Which the net alcohol slug was3.9 feet long. Thus the total slug of alcohol used in Run 2 was 5.2 feetand equal to that in Run 1. In this process, the slug traveled 30 feet,or approximately 86 percent of the total core length, beforedegeneration of the pure solvent slug, and the oil produced represented86 percent of the total volume of original oil plus the volume of oilinjected in the slug.

A fundamental principle of this invention is that the system is forcedto achieve miscibility at an oil concentration higher than that of theplait point. In practically all reservoirs the concentration of oil inthe stabilized zone will be less than 50 percent; hence, any lineconnecting the composition of the stabilized bank with the 100 percentsolvent vertex of the ternary diagram will intersect the binodal curveto the left of the plait point in any system in which the plait point isto the right of the peak of the binodal curve. It is imperative,therefore, that a ypredetermined volume of additional hydrocarbon beadded to the solvent prior to injection to induce a transition zonecomposition path that crosses the binodal curve to the right of theplait point.

The preceding description defines our invention of an improved miscible`slug process comprising the injection into a petroleum reservoir of amixed slug of solvent mixed with -a hydrocarbon, followed by a slug ofsolvent displaced by a scavenging Water flood. This order of stepsassures an expanding oil phase as the system achieves miscibility,thereby improving the relationship between slug size and distancetraveled to slug degeneration, increasing the recovery of reservoir oil,reducing the total volume of solvent required, and decreasing therequired injection pressure for the water flood portion of the process.

Therefore, we claim as our invention:

1. In a method for recovery of oil from a petroleum reservoir containingbrine and oil 'by introducing into said reservoir through an injectionwell a slug of amphipathic solvent and displacing said solvent nlwarilnlllrullillgww x...

wellrhynhejnjectionnoawateLiuto the reservoir at the injection well, theimprovement comprising injecting into the reservoir ahead of saidsolvent slug a mixture of said amphipathic solvent and a hydrocarbonthat is miscible with the reservoir oil, the amounts `of solvent andhydrocarbon in said mixture being such that said mixture adjusts theliquid concentrations in a transition zone ahead of said solvent slug ina manner such that as said solvent forms brineand toil-solvent phases in`said transition Zone said oil-solvent phase will increase in volume.

2. A method for recovery of oil from a petroleum reservoir containingoil and brine comprising introducing into said reservoir through aninjection well a mixed slug of an amphipathic solvent mixed with ahydrocarbon that is miscible with the reservoir oil, following saidmixture of solvent and hydrocarbon with a slug of amphipathic solventintroduced into said reservoir through said injection well, anddisplacing said amphipathic solvent slug through said reservoir toward aproducing well by injection of water into said reservoir whereby saidmixed slug forms a transition zone with said oil and brine ahead of saidamphipathic solvent slug, the amounts of solvent and hydrocarbon in saidmixed slug being such that said mixed slug adjusts the liquidconcentrations in said transition zone in a manner such that, as saidliquids achieve miscibility, an oil-solvent phase is formed whichincreases in volume and is displaced toward said producing well by saidamphipathic solvent slug.

3. A method according to claim 2 wherein said hydrocarbon that is mixedwith said amphipathic solvent is crude oil.

4. A method according to claim 2 wherein said hydrocarbon that -is mixedwith said amphipathic solvent is platformate.

5. A method according to claim 2 wherein said hydrocarbon that is mixedwith said amphipathic solvent is LPG.

6. A method according to claim 2 wherein said hydrocarbon that is mixedwith said amphipathic solvent is selected from the group consisting ofgasoline, kerosene, naphtha, LPG, benzene and platformate.

7. A method according to claim 2 wherein said hydrocarbon that is mixedwith said amphipathic solvent is a rened aliphatic hydrocarbon havingfrom three to twelve carbon atoms per mo1ecule.

8. A method according to claim 2 wherein said amphipathic solvent is amonohydroxy alcohol having from one to four carbon atoms per molecule.

9. A method according to claim 2 wherein said amphipathic solvent istertiary butyl alcohol.

10. A method according to claim 2 wherein said amphipathic solvent isisopropyl alcohol.

11. A method according to claim 2 wherein said amphipathic solvent isethyl alcohol.

12. A method according to claim 2 wherein said amphipathic solvent isselected from the group consisting of acetone, acetaldehyde, dioxane,ethylene oxide, alcohols having one to three carbon atoms per molecule,and tertiary butyl alcohol.

13. A method for recovery of oil from an oil-bearing formationcontaining oil and 'brine comprising displacing a low molecular Weighthydrocarbon liquid selected from the group consisting of kerosene andhydrocarbon liquids of lower average molecular weight than kerosene downan injection well penetrating said formation and through said formationfrom the injection well to a production well, following said lowmolecular weight hydrocarbon with a mixed slug of a mixture of anamphipathic solvent and a hydrocarbon that is miscible with theformation oil and with said preceding low molecular weight hydrocarbon,following said mixed slug with a slug of said amphipathic solvent, andfollowing said slug of amphipathic solvent with an aqueous liquid todisplace oil in the formation through said formation to the productionwell the amounts of solvent and hydrocarbon in said mixed slug beingsuch that said mixed slug adjusts the liquid concentrations in atransition zone ahead of said amphipathic solvent slug in a manner such`that as said solvent forms brineand oil-solvent phases in saidtransition zone said oil-solvent phase increases in volume.

14. A method according to claim 13 wherein said low molecular weighthydrocarbon liquid that is displaced down `the injection well and 4intothe formation ahead of said mixed slug is selected from the groupconsisting of gasoline, naphtha, LPG, benzene and platformate.

15. A method according to claim 13 wherein the amphipathic solvent isselected from the group consisting of methyl alcohol and ethyl alcohol.

References Cited by the Examiner UNITED STATES PATENTS 2,742,089 4/1956Morse 166-9 2,867,277 1/1959 Weinaug 166-9 3,101,781 8/1963 Connally166-9 FOREIGN PATENTS 696,524 9/-1953` Great Britain. 726,712 `3/ 1955Great Britain.

OTHER REFERENCES Taber, J. I. et al.: Mechanism of Alcohol Displacementof Oil from Porous Media. In Society of Petroleum Engineers Journal,vol. No. 3, September 1961, pp. to 207.

Burcik, E. J.: The Ternary Phase Diagram for Water, Isopropyl Alcohol,and Liquid Propane. The Pennsylvania State Univ. Mineral IndustriesExperiment Station, circular No. 61, Oct. 23-25, 1961, pp. 156 to 163.

CHARLES E. OCONNELL, Primary Examiner. C. H. GOLD, T. A. ZALENSKI,Assistant Examiners.

1. IN A METHOD FOR RECOVERY OF OIL FROM A PETROLEUM RESERVOIR CONTAININGBRINE AND OIL BY INTRODUCING INTO SAID RESERVOIR THROUGH AN INJECTIONWELL A SLUG OF AMPHIPATHIC SOLVENT AND DISPLACING SAID SOLVENT TOWARD APRODUCING WELL BY THE INJECTON OF WATER INTO THE RESERVOIR AT THEINJECTION WELL, THE IMPROVEMENT COMPRISING INJECTING INTO THE RESERVOIRAHEAD OF SAID SOLVENT SLUG A MIXTURE OF SAID AMPHIPATHIC SOLVENT AND AHYDROCARBON THAT IS MISCIBLE WITH THE RESERVOIR OIL, THE AMOUNTS OFSOLVENT AND HYDROCARBON IN SAID MIXTURE BEING SUCH THAT SAID MIXTUREADJUSTS THE LIQUID CONCENTRATIONS IN A TRANSITION ZONE AHEAD OF SAIDSOLVENT SLUG IN A MANNER SUCH THAT AS SAID SOLVENT FORMS BRINE- ANDOIL-SOLVENT PHASES IN SAID TRANSITION ZONE SAID OIL-SOLVENT PHASE WILLINCREASE IN VOLUME.